1. Field of the Invention
The present invention generally relates to drilling boreholes and, particularly, to a resistivity imager.
2. Description of the Related Art
In underground drilling applications, such as oil and gas exploration and recovery, a borehole is drilled into the earth. As a part of the drilling process, drilling mud is typically introduced into the borehole. One type of drilling mud is referred to as “oil-based” mud, while another is “water-based” mud. Other fluids may be found in a borehole as well. For example, boreholes may include formation fluids such as oil, gas, water, saline water as well as various combinations of these and other fluids. The various fluids found in a borehole can present complications for imaging of the formation.
One technique for imaging downhole formations is that of resistivity imaging. Resistivity imaging can be performed while drilling or at a later time. In general, resistivity imaging includes utilizing a resistivity instrument that provides a voltage to a formation and measures a current received from the formation. Utilizing Ohm's law, the resistivity of the formation is based on the relationship between the applied voltage and the measured current.
Many factors can affect the resolution of the resistivity imaging instruments. For example, instrument standoff (i.e., the gap between the wall of the borehole and the voltage source and/or current sensor), variability of the standoff, and variability of the electrical properties of the drilling mud as well as the formation can all affect resolution of the resistivity imaging instrument.
One particular challenging situation for imaging low resistivity formations, such as in the Gulf of Mexico, arises in the wells where the oil-based mud has been used as a drilling fluid. Oil-based mud is typically characterized by a very high value of resistivity. That is, total impedance, measured by a resistivity imaging instrument, primarily includes three sequentially connected impedances formed respectively by the formation, the drilling fluid, and the instrument measurement circuit itself. Typically, impedance of the instrument measurement circuit has been known and small compared to those of the formation and drilling fluid, and, therefore, could be easy accounted for or often neglected. Accordingly, sensitivity of the instrument to the changes in resistivity of the formation deteriorates as a contribution of the formation into the overall impedance goes down.
What are needed are techniques for enhancing resistivity images taken downhole. Preferably, the techniques provide improved image quality in the conditions of oil-based mud and low resistive formations.